Russel-Research-Method-in-Anthropology

Scales and Scaling 335
TABLE 12.7
The Data for the Interim Correlation
Person Total Score Item 1 Item 2 Item 3 . . Item 50
1 x x x x . . x
2 x x x x . . x
3 x x x x . . x
. . . . . . . .
. . . . . . . .
N x x x x . . x
With 50 items, the total score gives you an idea of where each person stands
on the concept you’re trying to measure. If the interitem correlation were perfect,
then every item would be contributing equally to our understanding of
where each respondent stands. Of course, some items do better than others.
The ones that don’t contribute a lot will correlate poorly with the total score
for each person. Keep the items that have the highest correlation with the total
scores.
You can use any statistical analysis package to find the interitem correlations,
Cronbach’s alpha, and the item-total correlations for a set of preliminary
scale items. Your goal is to get rid of items that detract from a high interitem
correlation and to keep the alpha coefficient above 0.80. (For an excellent stepby-step
explanation of item analysis, see Spector 1992:43–46.)
Testing for Unidimensionality with Factor Analysis
Factor analysis is a technique for data reduction. If you have 30 items in a
pool of potential scale items, and responses from a sample of people to those
pool items, factor analysis lets you reduce the 30 items to a smaller set—say,
5 or 6. Each item is given a score, called its factor loading. This tells you
how much each item ‘‘belongs’’ to each of the underlying factors. (See chapter
21 for a brief introduction to factor analysis and Comrey [1992] for more coverage.)
If a scale is unidimensional, there will be a single, overwhelming factor that
underlies all the variables (items) and all the items will ‘‘load high’’ on that
single factor. If a scale is multidimensional, then there will be a series of factors
that underlie sets of variables. Scale developers get a large pool of potential
scale items (at least 40) and ask a lot of people (at least 200) to respond
to the items. Then they run the factor analysis and select those items that load